请问有大佬做过喜树碱CPT与三光气、2-羟乙基二硫化物在DMAP催化下生成CPT-ss-OH的反应嘛？下面是我参考的文献部分，反应过程我注意了严格绝水，但是反应的转化率还是很低，比文献提到的产率为80%低很多，哪位大佬能给我一些合成方面的建议和改进方法嘛？
     
               CPT (350 mg, 1 mmol) and triphosgene (110 mg, 0.34 mmol) were dispersed in anhydrous DCM (50 mL), to which, 4-dimetheylaminopyridine (DMAP) (390 mg, 3.2 mmol) was added. The mixture was stirred at room temperature for 3 h. 2-Hydroxyethyl disulfide (HEDS) (0.77 g, 5 mmol) in THF (10 mL) was added. The solution was stirred at room temperature. The reaction process was monitored by TLC and stopped after the starting material was completely consumed. The reaction solution was diluted with DCM (200 mL), and washed with HCl (pH 1, 3 × 50 mL) and then with water (3 × 50 mL). The organic phase was dried with Na2SO4. After the solvent was removed under vacuum, CPT-SS-OH was purified by silica gel column chromatography (DCM/MeOH = 50:1 v/v) to give a slight yellow solid (438 mg, yiel  80%).


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